Centrifugal compressor

ABSTRACT

The present invention provides a centrifugal compressor that inhibits possible leakage while reducing a destabilizing fluid force generated in a seal to prevent the possible instable vibration of a rotor. The centrifugal compressor includes a casing, a rotor rotatably installed in the casing and having an impeller, and seals provided in the clearance between the casing and the rotor to prevent a fluid from leaking through the clearance from a high pressure side to a low pressure side. The rotor rotates to compress gas. For example, the balance piston seal is composed of a damper seal with a plurality of holes and a labyrinth seal with an annular parallel groove; the damper seal and the labyrinth seal are continuously provided. The damper seal is disposed on the high pressure side in a leakage flow direction. The labyrinth seal is disposed on the low pressure side in the leakage flow direction.

BACKGROUND OF THE INVENTION

The present invention relates to a seal structure for a centrifugalcompressor, and is suitable particularly for preventing unstablevibration of a rotor during a high-speed, high-pressure operation.

Centrifugal compressors compressing gas such as air are widely utilizedfor various machines. Inside of a casing of a centrifugal compressor, arotating shaft with an impeller installed thereon is rotatably supportedby bearings. Gas sucked through a suction port is compressed by rotationof the impeller and discharged through a discharge port. The gascompressed by the impeller is sealed by an impeller eye seal of animpeller eye, an inter-stage seal between stages of the impeller, and abalance piston seal provided in a last stage.

A labyrinth seal and a damper seal are known as conventional sealstructures. As shown in a sectional view of any impeller stage in acentrifugal compressor in FIG. 1 of JP Published Patent Application No.6-249186, the labyrinth seal structure has a large number of annularfins in the clearance between a rotor and a stator. Thus, a pressureloss in a fluid flowing through the top clearance between the fins andthe rotor reduces the leakage of the fluid.

The damper seal has a seal structure with a plurality of holes formed ina seal stator surface and is classified into a hole pattern seal, ahoneycomb seal, and the like. The hole pattern seal structure has alarge number of holes in the seal stator surface as shown in JPPublished Patent Application No. 6-249186, for example. Thus, a pressureloss in a fluid flowing through the clearance between the uneven sealstator surface and the rotor reduces the leakage of the fluid.Furthermore, the honeycomb seal is disclosed in, for example, JPPublished Patent Application Nos. 11-44201 and 2007-113458. Inparticular, FIG. 1(b) of JP Published Patent Application No. 11-44201clearly shows a honeycomb seal structure. The seal stator surface shownin FIG. 1(b) has a honeycomb structure with a large number of hexagonalshaped holes. Thus, a pressure loss in a fluid flowing by the unevenseal stator surface reduces the leakage of the fluid.

In the above-described seals, when the shaft is displaced in a radialdirection with the leakage flow velocity of the seal having acircumferential component, the circumferential pressure distribution inthe seal becomes asymmetrical. This results in a fluid force(hereinafter referred to as a “destabilizing fluid force) destabilizingthe rotor. In the worst case, the destabilizing fluid force causes therotor to vibrate unstably. In particular, if the rotor rotates at highspeed or there is a high differential pressure between the seal inletand outlet, the destabilizing fluid force is increased. As is wellknown, if the damper seal such as the hole pattern seal or the honeycombseal is used instead of the labyrinth seal, the unstable vibration ofthe rotor caused by the destabilizing fluid force can be stabilizedbecause the damper seal exerts a higher damping effect than thelabyrinth seal.

The labyrinth seal is excellent in the leakage prevention property.However, the increased discharge pressure of the centrifugal compressorincreases the destabilizing fluid force, thus reducing the vibrationstability of the rotor. The damper seal exerts a higher damping effectthan the labyrinth seal, thus stabilizing the vibration of the rotor.However, the damper seal is inferior in the leakage prevention property,thus reducing the efficiency of the compressor.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a centrifugalcompressor that can be stably operated under a high-speed andhigh-pressure condition, with possible leakage from a seal prevented.

To accomplish the object, the present invention provides a centrifugalcompressor comprising a casing, a rotor comprising a rotating shaftrotatably installed in the casing and an impeller installed on therotating shaft, and a seal preventing a fluid from flowing between astator and the rotor in the casing from a high pressure side to a lowpressure side, the impeller rotating to compress gas, wherein in theseal, a damper seal with a plurality of holes and a labyrinth seal withan annular parallel groove are continuously provided on a statorsurface.

Furthermore, preferably, in the seal, the damper seal is disposed on thehigh pressure side in a leakage flow direction of the seal, and thelabyrinth seal is disposed on a low pressure side in the leakage flowdirection of the seal.

Furthermore, preferably, in the seal, length of the damper seal in theleakage flow direction is set to at most half of that of the entireseal.

Furthermore, preferably, in the seal, the length of the damper seal inthe leakage flow direction is set equal to or larger than the length ofthe entire seal multiplied by 0.05.

According to the present invention, the damper seal and the labyrinthseal are continuously provided. Thus, the labyrinth seal can inhibitpossible leakage, while the damper seal can stabilize the rotor.Moreover, the damper seal is disposed on the high pressure side of theseal, whereas the labyrinth seal is disposed on the low pressure side.Thus, on the high pressure side, where a relatively strong destabilizingfluid force may be generated, the damper seal can exert a dampingeffect. Furthermore, on the high pressure side, corresponding to aleakage flow upstream side, the circumferential component of the leakageflow velocity can be reduced. This enables the rotor to be furtherstabilized. Additionally, the length of the damper seal in the leakageflow direction is set to at most half of that of the entire seal. Thisallows the rotor to be effectively stabilized, with possible leakagefrom the seal inhibited.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a perspective view showing, in detail, the structure of astator side of a balance piston seal according to an embodiment of acentrifugal compressor according to the present invention;

FIG. 2 is a sectional view showing the general structure of anembodiment of the centrifugal compressor according to the presentinvention;

FIG. 3 is an enlarged partially sectional view showing the inside of anarea A in FIG. 2;

FIG. 4 is a diagram showing the relationship between a seal structureratio (the rate of the entire seal length accounted for by the length ofa hole pattern seal) and a seal instability indicator according to theembodiment of the centrifugal compressor according to the presentinvention;

FIG. 5 is a diagram showing the relationship between the rate of theentire seal length accounted for by the length of the hole pattern sealand the leakage flow rate of the entire seal according to the embodimentof the centrifugal compressor according to the present invention; and

FIG. 6 is a sectional perspective view showing a variation of thestructure of the stator side of the balance piston seal according to theembodiment of the centrifugal compressor according to the presentinvention;

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described below withreference to the drawings.

FIG. 1 is a perspective view showing, in detail, the structure of abalance piston seal 14 in an embodiment of a centrifugal compressor 20according to the present invention. An illustrated damper seal is a holepattern seal 15 made up of a large number of holes 16. FIG. 2 shows thegeneral structure of a centrifugal compressor 20 according to thepresent invention. FIG. 3 is a diagram showing an area A in FIG. 2.

In FIG. 2, the centrifugal compressor 20 includes a casing 1 (stationarymember), a rotating shaft 2 rotatably provided in the casing 1, and arotor 4 having a multi-stage (in FIG. 2, seven-stage) impeller 3installed on the rotating shaft 2. The casing 1 includes a suctionchannel 5 through which gas is introduced into a first stage of theimpeller 3, a diffuser 6 that converts kinetic energy from each stage ofthe impeller 3 into pressure energy, a return channel 7 through whichcompressed gas from the diffuser 6 is introduced into the followingstage of the impeller 3, and a discharge channel 8 through which gasfrom the final stage of the impeller 3 is discharged.

The rotating shaft 2 of the rotor 4 is rotatably supported by radialbearings 9 provided at a suction-side (the left side of FIG. 2) end anda discharge-side (the right side of FIG. 2) end. A thrust bearing 10 isalso provided at the suction-side end of the rotating shaft 2 to receivea thrust load. A balance piston 11 is provided at the discharge-side endto offset the thrust load. Additionally, a driver such as a motor (notshown in the drawings) is coupled to the discharge-side end of therotating shaft 2. The driver drives the rotor 4, which is thus rotated.The rotation of the rotor 4 allows gas to be sucked through the suctionchannel 5, sequentially compressed by the multi-stage impeller 3, andfinally discharged through the discharge channel 8.

An impeller eye seal 12 is provided in the clearance between each stageof the impeller 3 and the impeller eye 21. The impeller eye seal 12inhibits gas from the impeller 3 from passing through the clearance andback to the inlet of the impeller 3 (see FIG. 3). Furthermore, aninter-stage seal 13 is provided between each stage of the impeller 3 andthe succeeding stage of the impeller 3 in the clearance between theinter-stage 22 of the rotor 4 and the casing 1. The inter-stage seal 13inhibits gas from the return channel 7 from passing through theclearance and back to the outlet of the preceding stage of the impeller3. Additionally, as shown in FIG. 3, a balance piston seal 14 isprovided in the clearance between the balance piston 11 of the rotor 4and the casing 1. The balance piston seal 14 inhibits high-pressure gasfrom the final stage of the impeller 3 from leaking to a low pressureportion.

The balance piston seal 14 is composed of two parts shaped like the twohalves of a cylinder. FIG. 1 shows one of the halves. In FIG. 1, thebalance piston seal 14 has a hole pattern seal 15 located on a statorsurface opposite to the balance piston 11 and on a high pressure side ofa leakage flow direction and composed of a plurality of holes 16, and alabyrinth seal 17 located on a low pressure side of the leakage flowdirection and composed of annular parallel fins 18 and annular parallelgrooves 19. The length of the hole pattern seal 15 in the leakage flowdirection is defined as Lh. The length of the labyrinth seal 17 in theleakage flow direction is defined as Ll.

FIG. 4 is a diagram showing an example of the relationship between therate of the length of the hole pattern seal 15 in the leakage flowdirection accounted for by the entire seal length in the leakage flowdirection (the rate is hereinafter referred to as a seal structureratio) and a seal instability indicator. The seal structure ratio iscalculated to be Lh/(Lh+Ll) where Lh denotes the length of hole patternseal 15 and Ll denotes the length of the labyrinth seal. The sealinstability indicator is calculated to be Kxy/(Cxx.ω) that is the ratioof the stiffness coefficient Kxy of the seal obtained by dividing thedestabilizing fluid force by the radial displacement of the rotatingshaft 2 to the product of the damping coefficient Cxx of the seal forthe radial displacement and the eigen angular frequency ω of the rotor4. A decrease in seal instability indicator improves the stability ofthe rotor. FIG. 4 shows that a seal structure ratio of higher than 0.5allows the hole pattern seal to exert almost constant stabilizingeffect. Furthermore, a seal structure ratio close to 0.0 rapidlyincreases the seal instability indicator. Thus, to ensure the stabilityof the seal, the seal instability indicator needs to be kept at 0.6 orless. Desirably, the seal structure ratio is correspondingly kept atabout 0.05 or more.

FIG. 5 is a diagram showing an example of the relationship between theseal structure ratio and the leakage flow rate ratio of the entire sealobserved when the entire seal length is constant. The leakage flow rateratio is the leakage flow rate expressed in terms of ratio where theleakage flow rate is set to 1 when the seal structure ratio is 0. Theleakage flow rate increases consistently with the seal structure ratio,that is, with the rate of the hole pattern seal. FIGS. 3 and 4 show thatthe seal structure ratio may be set to at most 0.5 in order to improvethe stability of the rotor while inhibiting a possible increase inleakage flow rate.

At the same seal length, the above-described structure enables areduction in leakage flow rate compared to a structure in which thebalance piston seal 14 is entirely composed of the hole pattern seal 15.Moreover, a stronger destabilizing fluid force is generated on the highpressure side of the seal. However, in the present embodiment, the holepattern seal 15, exerting the damping effect, is disposed on the highpressure side. This improves the stability of the rotor. Furthermore,the hole pattern seal 15 disposed on the high pressure side,corresponding to the upstream side of the leakage flow, enables areduction in the circumferential component of the velocity of theleakage flow toward the labyrinth seal 17, located on the low pressureside, that is, the downstream side. Thus, the destabilizing fluid forcein the labyrinth seal 17 can be reduced, allowing the possible unstablevibration of the rotor 4 to be inhibited.

In the example described in the embodiment, the hole pattern seal 15with the circular holes 16 formed therein at equal intervals is disposedon the high pressure side of the balance piston seal 14 as a damperseal. However, the present invention is not limited to this aspect. Forexample, the damper seal may be a honeycomb seal composed of hexagonalshaped holes or a seal composed of triangular or rectangular shapedholes.

Furthermore, as shown in FIG. 1, the present invention is not limited tothe pattern in which the holes 16 are closely arranged at equalintervals. For example, as shown in FIG. 6, the seal may have a portionin which no hole 16 is provided in part of the axial direction.Moreover, the intervals of the holes may be varied in thecircumferential direction.

Additionally, in the above-described example, the hole pattern seal 15and the labyrinth seal 17 are composed of the continuous parts. However,the present invention is not limited to this aspect. For example, thehole pattern seal 15 and the labyrinth seal 17 may be made of separateparts, which may then be combined together.

The balance piston seal 14 has been described by way of example.However, similar effects can be exerted by applying the presentinvention to the impeller eye seal 12 and the inter-stage seal 13.

1. A centrifugal compressor comprising a casing, a rotor comprising arotating shaft rotatably installed in said casing and an impellerinstalled on said rotating shaft, and a seal preventing a fluid fromleaking between said casing and said rotor from a high pressure side toa low pressure side, wherein said seal has a damper seal with aplurality of cavity formed on a seal surface and a labyrinth seal with aplurality of annular grooves.
 2. The centrifugal compressor according toclaim 1, wherein said labyrinth seal is provided juxtaposed to saiddamper seal.
 3. The centrifugal compressor according to claim 2, whereinsaid damper seal is disposed on a high pressure side in a leakage flowof said seal, and said labyrinth seal is disposed on a low pressure sidein the leakage flow of said seal.
 4. The centrifugal compressoraccording to claim 3, wherein a length of said damper seal in a leakageflow direction is set to at most half of that of said seal.
 5. Thecentrifugal compressor according to claim 4, wherein a length of saiddamper seal in a leakage flow direction is set to or larger than alength of said seal multiplied by 0.05.